1. Field of the Invention
The present invention relates to a gas sensor (hereinafter also referred to as a “sensor”) such as an oxygen sensor for detecting the concentration of oxygen in a gas to be measured, such as exhaust gas discharged from, for example, an internal combustion engine.
2. Description of the Related Art
Patent Document 1 discloses a typical example of such a gas sensor, which has the structure shown in FIG. 11. This oxygen sensor 1 is composed of a detection element (hereinafter also referred to as an “element”) 21, a metal housing 11, etc. The detection element 21 is formed from a cup-shaped (tubular) solid electrolyte having an inner electrode (layer) and an outer electrode (layer) formed on inner and outer surfaces thereof, respectively, and a closed front end (lower end in FIG. 11). The metal housing 11 holds the detection element 21 therein, and the oxygen sensor 1 is mounted to an exhaust gas pipe of the internal combustion engine via the metal housing 11. The inner electrode (reference electrode) on the inner circumferential surface (inner wall surface) of the detection element 21 is exposed to a reference gas (ambient atmosphere), and the outer electrode (measurement electrode) on the outer circumferential surface (outer wall surface) of the detection element 21 is exposed to an exhaust gas. In this manner, an electromotive force is generated between the two electrodes in accordance with a difference in oxygen concentration between the inner and outer surfaces of the detection element 21. A signal induced by this electromotive force is output to a control circuit, and is used to detect the concentration of oxygen in the exhaust gas and control the air-fuel ratio.
In the sensor 1 shown in FIG. 11, terminal leads 71 and 91 are connected to the inner and outer electrodes (hereinafter also referred to as “electrodes”) formed on the inner and outer circumferential surfaces of the detection element 21; and lead wires 41 for signal output are connected to the rear ends (upper ends in FIG. 11) of the terminal leads 71 and 91. Of these terminal leads 71 and 91, the terminal lead (output terminal) 71 connected to the electrode formed on the inner circumferential surface of a rear end portion of the detection element 21 has a leading end connecting portion 73, as shown in FIG. 2A. This leading end connecting portion 73 is a cylindrical portion (a tubular portion having a C-shaped or Landolt-ring-shaped transverse cross section) which is formed by, for example, bending a metal plate and which has a slit-like opening M along the generating line. The leading end connecting portion 73 is brought into press contact with and is connected to the inner electrode by means of press-fitting the leading end connecting portion 73 into the interior of the detection element 21 from the rear end thereof. As shown in FIG. 3A, the terminal lead (ground terminal) 91 connected to the electrode formed on the outer circumferential surface of the element 21 has a leading end connecting portion 93 at its front end portion. This leading end connecting portion 93 is a cylindrical portion which is formed by, for example, bending a metal plate and which has a slit-like opening M along the generating line. The leading end connecting portion 93 is brought into press contact with and is connected to the outer electrode by means of press-fitting the leading end connecting portion 93 onto the outer circumference of the detection element 21 from the rear end thereof. These terminal leads 71 and 91 include trailing end connecting portions 75 and 95 and intermediate portions 74 and 94, respectively. The trailing end connecting portions 75 and 95 are connected to respective distal ends of the lead wires 41 by means of crimping. The intermediate portions 74 and 94 connect the cylindrical leading end connecting portions 73 and 93 and the trailing end connecting portions 75 and 95, respectively. The terminal leads 71 and 91 are configured such that after being press-fitted into and onto the detection element 21, respectively, the leading end connecting portions 73 and 93 are pressed against the inner and outer electrodes of the detection element 21 by means of their spring forces, to thereby electrically connect thereto.
Assembly of the sensor 1, involving such electrical connection, has been performed as follows. An element-side subassembly (main-body-side assembly) 201 in which the element 21 is fixedly disposed inside the metal housing 11 assuming a tubular shape by use of a seal member 53 or the like as shown on the left side of FIG. 12 is assembled along with a terminal-lead-side subassembly 301 in which the terminal leads 71 and 91 are connected to the distal ends of the lead wires 41 within a tubular protection sleeve 31 (in the form of a stepped cylindrical tube) as shown on the right side of FIG. 12. The terminal leads 71 and 91 of the terminal-lead-side subassembly 301 are inserted into a plurality of insertion holes 113 provided in a cylindrical-columnar separator 111 formed of an insulating material and extending through the separator in the front-rear direction. This structure secures electrical insulation between the terminal leads 71 and 91 and between the terminal leads 71 and 91 and the protection sleeve 31, serving as a casing. FIG. 13 is a view of the separator 111 of the terminal-lead-side subassembly 301 on the right side of FIG. 12, as viewed from the side where the front end (lower end in FIG. 12) thereof is located. As shown in FIG. 13, the tubular leading end connecting portions 73 and 93 are disposed so that their centers coincide with the center of a front end surface 112 of the separator 111 or are located in the vicinity of the center. At the front end of the separator 111, the leading end connecting portions 73 and 93 form a so-called double circle. Notably, in FIG. 12, a pole-shaped heater 61 extending along an axis G is provided below the terminal-lead-side subassembly 301 and held within the leading end connecting portion 73 of the terminal lead 71 to project from the leading end connecting portion 73. However, the heater 61 will not be described here.
The sensor 1 is assembled as follows. After the centers (axis G) of the leading end connecting portions 73 and 93 of the terminal leads 71 and 91 of the terminal-lead-side subassembly 301 are made exactly or generally coincident with the axis G of the element 21 of the element-side subassembly 201 as shown in FIG. 12, these subassemblies are axially moved toward each other. Thus, the leading end connecting portions 73 and 93 of the terminal leads 71 and 91 are simultaneously fitted onto the inner and outer circumferences of the rear end 25 of the detection element 21. Notably, in order to facilitate this fitting, chamfers 26 and 27 are formed at respective corners between the rear end 25 of the element 21 and the inner and outer circumferential surfaces thereof. Meanwhile, a taper portion 84 and outwardly-expanded teeth 98 are provided at the front ends of the leading end connecting portions 73 and 93 of the terminal leads 71 and 91 so as to serve as guides. Simultaneously with fitting of these leading end connecting portions, a front end portion of the protection sleeve 31 is fitted onto a rear-end side thick cylindrical portion 17 of the metal housing 11.
The terminal-lead-side subassembly 301 used for assembly of such a sensor is generally assembled through the following steps. The distal end portions of the lead wires 41 are passed through a seal member 131 and then through the insertion holes 113 of the separator 111, and the trailing end connecting portions (connector portions) 75 and 95 at the rear ends of the terminal leads 71 and 91 are connected to the distal ends of the passed lead wires 41 by means of crimping. Subsequently, rear end (proximal end) portions of the lead wires 41 are pulled toward the rear of the separator 111 and the seal member 131 (upward in FIG. 12) until the leading end connecting portions 73 and 93 of the terminal leads 71 and 91 engage the front end surface 112 of the separator 111, whereby positioning of the terminal leads 71 and 91 is completed.
Meanwhile, in the course of assembling the terminal-lead-side subassembly 301, the tubular leading end connecting portions 73 and 93 of the terminal leads 71 and 91 must be positioned, with a predetermined accuracy, at predetermined positions corresponding to the rear end of the element 21 of the element-side subassembly 201 on the front end surface 112 of the separator 111. If the leading end connecting portions 73 and 93 are not positioned in such a manner, the leading end connecting portions 73 and 93 cannot be smoothly fitted onto the inner and outer circumferential surfaces of the rear end 25 of the element 21 at the time of assembling the subassemblies 201 and 301 toward each other. Conventionally, the element-side subassembly 201 has been relatively easily assembled as designed. However, in assembling the terminal-lead-side subassembly 301, it has been difficult to accurately position the tubular leading end connecting portions 73 and 93 of the terminal leads 71 and 91 at designated positions on the front end surface 112 of the separator 111. This is because the intermediate portions 74 and 94 of the terminal leads 71 and 91 allow for a so-called swaying motion; i.e., sway in the insertion holes 113 of the separator 111.
Therefore, in the above-mentioned sensor described in Patent Document 1, the terminal leads 71 and 91 and the insertion holes 113 of the separator 111 into which the terminal leads 71 and 91 are inserted are configured to have the following relation. In order to stably position the terminal leads 71 and 91 within the insertion holes 113 of the separator 111, as shown in FIGS. 2 and 3, engagement portions 76 and 96 extending in the front-rear direction are respectively formed at the intermediate portions 74 and 94 between the leading end connecting portions 73 and 93 and the trailing end connecting portions 75 and 95 of the terminal leads 71 and 91, and spring tabs 77 and 97 are centrally formed in the engagement portions 76 and 96. The spring tabs 77 and 97 are formed through punching and bending so as to project from second faces 76a and 96a of the engagement portions 76 and 96 and have end portions 77a and 97a. By virtue of this configuration, when the intermediate portions 74 and 94 of the terminal leads 71 and 91 are inserted into the insertion holes 113, as shown in FIGS. 12 and 14, the spring tabs 77 and 97 are pressed against the wall surfaces of the holes 113 by means of spring force, while pressing the engagement portions 76 and 96 against the opposite wall surfaces of the holes 113. Such pressing prevents the intermediate portions 74 and 94 from swaying within the holes 113.
As shown in, for example, FIG. 3-b of Patent Document 1, the insertion holes 113 of the separator 111 are formed at predetermined positions around the axis to pass through the separator in the front-rear direction. However, each hole (the lead-wire insertion hole 31a in FIG. 3-b of Patent Document 1) has a generally triangular transverse cross section which extends toward the axis G to form an apex. Therefore, in the above-described sway prevention means, as shown in FIG. 14, the spring tabs 77 and 97 are pressed against the wall surfaces of the holes 113 remote from the axis G, and the first faces 76b and 96b of the engagement portions 76 and 96 are pressed, at their opposite lateral edge corners 76c and 96c, against the opposite wall surfaces of the holes 113. That is, the terminal leads 71 and 91 are positioned within the insertion holes 113 in this manner, and the leading end connecting portions 73 and 93 at their front ends are positioned on the front end surface 112 of the separator 111 as shown in FIG. 13.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2004-53425
3. Problems to be Solved by the Invention
The above-described means for preventing swaying of the terminal leads 71 and 91 raises the following problem. In the course of assembling the terminal-lead-side subassembly 301, the terminal leads 71 and 91 are inserted into the insertion holes 113 of the separator 111, and the leading end connecting portions 73 and 93 at their front ends are positioned and disposed on the front end surface 112 of the separator 111. However, this assembly operation cannot be carried out easily, or a large amount of labor (man-hours) is required for such positioning. That is, the dimensions of relevant portions of the holes 113 and the terminal leads 71 and 91 are set so that when the leading end connecting portions 73 and 93 engage the front end surface 112 of the separator 111, the centers of the leading end connecting portions 73 and 93 are located at a predetermined position on the front end surface 112. However, in assembling the terminal-lead-side subassembly 301, the desired positioning cannot be achieved by merely inserting the terminal leads 71 and 91 into the insertion holes 113 of the separator 111 and pulling out the lead wires 41 connected to the trailing end connecting portions 75 and 95 at the rear ends of the terminal leads 71 and 91 toward the proximal end side. Accordingly, during assembly of the terminal-lead-side subassembly 301, a worker has to actively adjust the positions of the leading end connecting portions 73 and 93 so that they are disposed at predetermined positions on the front end surface 112 of the separator 111.
Further, even when the leading end connecting portions 73 and 93 are correctly disposed at the desired positions during assembly, as shown in FIG. 15, the positions of the leading end connecting portions 73 and 93 may change due to various external forces, vibration, or the like, applied to the leading end connecting portions 73 and 93 before the two subassemblies are assembled together. Such a change in position occurs because, as viewed in a transverse cross section, both the intermediate portions 74 and 94 of the terminal leads 71 and 91 can easily move to rotate within the insertion holes 113 of the separator 111. Thus, the leading end connecting portions 73 and 93 move in a swinging fashion about the intermediate portions 74 and 94 when an external force or the like is applied thereto. The reason for this movement is that each insertion hole 113 of the separator 111 assumes a generally triangular shape in the transverse cross section as described above. That is, in the transverse cross section, each hole 113 has opposite inclined wall surfaces, the distance therebetween decreasing toward the axis G. Meanwhile, the opposite lateral edge corners 76c and 96c of the engagement portions 76 and 96 of the intermediate portions 74 and 94 are engaged with and supported by the inclined wall surfaces of the corresponding holes 113, and such support is unstable. In the case where such support is employed, when an external force is laterally applied to the leading end connecting portion 73 (93) about the intermediate portion 74 (94), the intermediate portion 74 (94) readily rotates within the corresponding hole 113. Even when the intermediate portion 74 (94) rotates by a small amount, the leading end connecting portion 73 (93) at the front end swings a great distance in the transverse cross section, and its position changes considerably.
The above-described change in position may cause an assembly related problem in that when the terminal-lead-side subassembly 301 and the element-side subassembly 201 are assembled together, the leading end connecting portions 73 and 93 of the terminal leads 71 and 91 collide with the rear end portion of the detection element 21, and cannot be fitted smoothly. In such a case, the cylindrical portions, which constitute the leading end connecting portions 73 and 93, deform (for example, are crushed), chipping occurs at the rear end portion of the detection element 21, or fitting failure (connection failure) occurs, whereby the production yield of the sensor decreases.